Nitrous oxide (N2O) is a naturally occurring greenhouse gas with a warming potential per molecule of approximately 290 times that of a carbon dioxide molecule. Pure N2O is known to be explosive. Several industrial processes generate N2O as a by-product such as the preparation of adipic acid from cyclohexanone/cyclohexanol and nitric acid (HNO3). Adipic acid is a starting material in the manufacture of nylon 66, which has widespread use throughout the world as a component in, for example, home furnishings (carpets, upholstery, bedspreads and the like), apparel, tire cord, conveyor belts, seat belts, air bags, parachutes, ropes and nets, sleeping bags, tarpaulins, tents and auto parts. The adipic acid is typically prepared in a two-stage process, where the first stage involves the oxidation of cyclohexane to a cyclohexanone/cyclohexanol mixture, and the second stage involves oxidation of this mixture with nitric acid to produce adipic acid. Nitrous oxide, nitrogen oxides (NOx, collectively nitrogen monoxide (NO) and dioxide (NO2)) and water (H2O) are the chief by-products generated during this oxidation reaction. Notably, NOx is also a pollutant and its emission is subject to strict regulations.
Current methodologies practiced by large scale generators of N2O for conversion of the N2O to a more environmentally friendly form include (i) catalytically decomposing the N2O to nitrogen gas (N2) and oxygen gas (O2) which are released into the atmosphere; or (ii) treating a N2O-containing offgas with natural gas in a thermal reduction unit to form nitrogen gas, carbon dioxide and water (4 N2O+CH4→4 N2+CO2+2 H2O), which is then released into the atmosphere.
Both of these protocols have the drawback of converting the N2O to a non-useful form (nitrogen gas) which is discarded. There is, however, an ongoing need to recover and, if possible, to recycle or use in an economically effective manner the N2O and NOx components contained in an industrial waste gas or an offgas. U.S. Pat. No. 5,429,811 and Japanese Published Patent Application No. S61-257940 represent examples of conventional technologies for the thermal decomposition of N2O to NOx.
The present invention provides a significant improvement over conventional techniques for the preparation of nitric acid from N2O (via a NOx intermediate) and is especially effective when conducted on an industrial-scale. In the particular application of the present invention where the source of the initial N2O-containing reactant composition is the offgas from the production of adipic acid, the conversion of the N2O in the offgas to NOx and ultimately to nitric acid represents an efficient and safe recycling of the nitric acid starting material used in the production of adipic acid. This process is more cost effective than existing processes where the N2O by-product is destroyed by conversion to either unusable N2 gas or to nitric acid under reaction conditions not readily amenable to industrial level scale-up.